1. Field of the Invention
The present invention relates to a reinforced fiber sheet for use in repairing and/or reinforcing concrete structures. More particularly, the invention relates to a reinforced fiber sheet containing substantially no adhesive resin layer and comprising a net-shaped fabric substrate and reinforcing fibers uni-directionally and uniformly arranged thereon.
2. Description of the Prior Art
The The work life of concrete structures that contain concrete as the main structural material, such as ferro-concrete bridges, elevated expressways, tunnels, buildings, etc., is considered to be semi-permanent. Moreover, various post-construction side effects have been known to sometimes occur with these types of structures. These side effects result from the neutralization of the materials used, cracks due to rust and expansion of the iron skeleton, surface peeling, structural defects, or the use of low grade concrete and other construction materials. Furthermore, such side effects may also result from poor quality construction, traffic overload or the accumulation of an increased traffic load, and chemical reactions with salts and contaminated materials while under construction.
Because of such side effects, post-construction repairing/reinforcing processes may often be required, such as methods for reinforcing with steel panels, reinforced fiber sheets or with an appropriate combination thereof.
The reinforcement process of the prior art usually utilizes reinforced fiber sheets for repairing/reinforcing concrete structures. This fiber reinforcing method comprises the steps of placing an anchor plate (2) on the surface of a concrete structure to be repaired/reinforced and fixing it to the concrete structure with a bolt (3), attaching a reinforced fiber sheet (4) to both sides and bottom of the structure and fixing the sheet to the plate with a bracket (5), as shown in FIG. 1a. An alternative method may comprise of attaching a reinforced fiber sheet (4) to both sides and bottom of the structure, but without an anchor plate on the surface of the concrete structure, and directly fixing the sheet to the concrete structure with a bolt (6), as shown in FIG. 1b.
For example, a reinforced fiber sheet as disclosed in EP 441519 A1, which is shown in FIGS. 2a and 2b, may be applied to the above fiber reinforcing processes. As shown in FIG. 2a, the reinforced sheet (4) comprises a substrate (a) and reinforcing fibers (c) uni-directionally arranged thereon through an interposed adhesive layer (b) between said substrate and reinforcing fibers. Alternatively, the sheet may comprise a first and a second substrate and reinforcing fibers uni-directionally interposed between said substrates by means of adhesive layers, as shown in FIG. 2b.
As described above, conventional reinforcing sheets (4,5) generally include an integrated composite consisting of a substrate and reinforcing fibers uni-directionally arranged on either side thereof through an adhesive resin layer in a film shape between said substrate and reinforcing fibers. That is, conventional reinforced sheets essentially use an adhesive resin film in order to hold the unidirectional arrangement of reinforcing fibers on the substrate. Said adhesive resin film melts and permeates into the reinforcing fibers to form an adhesive resin layer over the fibers when the composite is compressed with heating to obtain a reinforced fiber sheet. Such a reinforced fiber sheet is referred to as a resinous reinforced fiber sheet, i.e. a prepreg.
When repairing and/or reinforcing concrete structures using such conventional resinous reinforced fiber sheets, a liquid cold-setting matrix resin is first applied to the surface of the concrete structure and the reinforced fiber sheet is then attached thereto.
Thereafter, the matrix resin penetrates into the reinforcing fibers of the sheet. Due to the adhesive resin layer that is formed over the reinforcing fibers, however, the matrix resin penetrates into the reinforcing fibers too slowly. That is, the above resinous reinforced fiber sheet contains sticky adhesive resin, which acts as a barrier against the permeation into the reinforcing fiber of the matrix resin that has been applied to the surface of the concrete structure undergoing repair/reinforcement.
Further, since the adhesive resin layer that is formed over the reinforcing fibers increases the structural density of the reinforcing fibers, it is difficult for matrix resin to permeate into the reinforcing fibers. Thus, the reinforced fiber sheet is liable to peel away from the concrete structure. Also, since it is difficult to remove air bubbles from the applied matrix resin, additional processing is required to acquire the desired adhesive strength.
Consequently, the use of the conventional resinous reinforced fiber sheet in repairing and/or reinforcing concrete structures undesirably lengthens the time of the work process and diminishes workability.
In addition, compression process, for example rolling process, are used to press the matrix resin into the reinforced fiber sheet. However, since this requires compression very high-powered, distortion in the shape of the reinforced fiber sheet can easily occur, resulting in a weakening of the reinforced sheet.